224 THE MORPHOLOGY OF THE COMMON CRAYFISH. 
on the one hand, and the cephalic integument on the 
other. 
The mesoblastic cells give rise to the layer of con- 
nective tissue which forms the deeper portion of the 
integument, and to that which invests the alimentary 
canal; to all the muscles; and to the heart, the. vessels, 
and the’corpuscles of the blood. The heart appears 
very early as a solid mass of mesoblastic cells in the 
tergal region of the thorax, just in front of the origin 
of the abdomen (figs. 57, 58, 59, h). It soon be- 
comes hollow, and its walls exhibit rhythmical con- 
tractions. 
The branchie are, at first, simple papille of the integu- 
ment of the region from which they take their rise. 
These papille elongate into stems, which give off lateral 
filaments. ‘The podobranchie are at first similar to the 
arthrobranchie, but an outgrowth soon takes place near 
the free end of the stem, and becomes the lamina, while 
the attached end enlarges into the base. 
The renal organ is stated to arise by a tubular involu- 
tion of the epiblast, which soon becomes convoluted, and 
gives rise to the green gland. 
The central nervous system is wholly a product of the 
epiblast. The cells which lie at the sides of the longi- 
tudinal groove already mentioned (fig. 58, mg), grow in- 
wards, and give rise to two cords which are at first 
separate from one another and continuous with the rest 
of the epiblast. At the front end of the groove a 
